PROJECT SUMMARY/ABSTRACT Unwanted immune responses by mast cells contribute to the symptoms of allergies and asthma. In the proposed research we seek to harness members of the Siglec family of inhibitory receptors to suppress allergen mediated IgE dependent activation and degranulation of human mast cells, and desensitize them to subsequent antigen challenge. To this end we will employ Siglec tolerizing allergenic liposomes (STALs) that display both an allergen and synthetic high affinity glycan ligand of a Siglec expressed on mast cells. When STALs encounter a mast cell pre-sensitized with an allergen specific IgE bound to the high affinity IgE receptor ?RI), the glycan ligand will recruit the inhibitory Siglec to the immunological synapse. While liposomes with antigen alone will powerfully activate the cells, the glycan ligand on STALs is hypothesized to recruit the inhibitory Siglec and dampen or suppress activation and degranulation. In this project we will focus on two Siglecs on human mast cells, namely CD33 and Siglec-8. We will assess the impact of STALs for desensitizing human mast cells in models of passive cutaneous and passive systemic anaphylaxis using transgenic mice with mast cells expressing a human Siglec (Siglec-8 and CD33) and a human Fc?RI receptor, and humanized mice engrafted with human CD34+ stem cells that populate the mouse with human mast cells. A portion of our effort will also be devoted to improving the specificity of the synthetic ligand for Siglec-8 and develop a novel ligand for the Siglec-6 receptor, a mast cell target for Project 1. (Fc 0